How to better track and manage your cellular data usage

If you've ever gone over the data allowance on your iPhone, you aren't alone.
It's a problem a lot of smartphone owners run into. From streaming music to watching YouTube videos, we all love to consume data. Unfortunately with capped data plans being the norm nowadays, it's easy to get hit with overage charges. That's exactly the problem an iMore reader is having:

Every once in a while I find myself going over what I'm allowed on my iPhone data plan. Is there an easier way to stay on top of this other than checking my carrier's app or getting text updates?